A number of light guiding devices are known. These devices are employed for a range of functions including illumination, backlighting, signage and display purposes. Typically, the devices are constructed from an injection moulded or machined transparent plastic component, where a light source, such as a fluorescent lamp or a plurality of light emitting diodes (LEDs), is integrated by means of mechanical attachment at the edge of the transparent plastic component.
Common to all of these devices is the fact that light from the light source is guided through a transparent guide, typically made of plastic, by total internal reflection. For backlighting applications, light is emitted in a substantially perpendicular direction to that of the direction of propagation of the light within the transparent guide. This is achieved through the light being directed so as to interact with scattering structures or films located within, or on the surface of, the transparent guide.
The integration of fluorescent lamps or LEDs to the edge of the transparent light guide is not a straightforward process and thus significantly increases the complexity of the production process for these devices. Achieving a good coupling of the light source and the light guide is essential to the optical performance of the device. In addition, edge coupling of the light sources renders these components susceptible to mechanical damage during both the production process and the normal use of the device.
In seeking to provide thin direct lit backlights, it is preferable to have light emitted into the plane of the light guide. Further benefit may be obtained if the light sources are distributed across the panel, so minimising the length of guiding in the light guide. This has the benefit of creating a thin and efficient backlight but has the disadvantage of compromising the light uniformity. For example, this may be through the creation of dark spots and/or more intense areas of light above or in the vicinity of the light source. Preferably, these dark spots and/or more intense areas of light should not be visible or, at least, reduced in appearance in order to provide at least acceptable, and more preferably, improved light uniformity. Existing solutions to this problem tend to add considerable thickness to the backlight.
Many backlights fall into the categories of “edge-lit” or “direct-lit”. These categories differ in the placement of the light sources relative to the output of the backlight, where the output area defines the viewable area of the display device. In edge-lit backlights, one or more light sources are disposed along an outer border or edge of the backlight construction outside the zone corresponding to the output area. The light sources typically emit light into a light guide, which has length and width dimensions of the order of the output area and from which light is extracted to illuminate the output area. In direct-lit backlights, an array of light sources is disposed directly behind the output area, and a diffuser is placed in front of the light sources to provide a more uniform light output. Some direct-lit backlights also incorporate an edge-mounted light, and are thus illuminated with a combination of direct-lit and edge-lit illumination.
Apart from producing light, the light sources also produces heat. If the heat dissipates into a liquid crystal panel for example, the display quality may be adversely affected. Because the transfer of heat to the liquid crystal panel is unlikely to be uniform, the liquid crystal layer within the liquid crystal panel may be subjected to different degrees of heating. This non-uniform distribution of heat may also affect the switching of thin film transistors inside the liquid crystal display and the overall display quality of the liquid crystal panel may deteriorate. There are numerous challenges to overcome in thermally managing a backlight unit for use in combination with displays. Addressing issues surrounding thermal management should not result in an unacceptable increase in weight or thickness or compromise the mechanical integrity of the backlight unit. In addition, the optical performance of the backlight unit should not be significantly compromised.
Other challenges facing display manufacturers, such as those incorporating large area LED Back Light Units (BLUs) include producing a thin and efficient device which enables 2-d or 3-d spatial dimming to support high display performance and reduced power consumption. This has proved problematic for both edge-lit and direct lit devices and has typically resulted in thicker backlight devices. 2-d dimming relates to when the image content of the display is achieved by only switching on selected areas of the backlight which match or correspond to the desired image thus resulting in significant power reduction. 3-d dimming further incorporates the use of colour.
The beam angle of light emerging from a backlight and the uniformity of said beam angle are important in determining the efficiency and the viewing angle of the display that is being illuminated. Some degree of control over the range of light output angles provided by the backlight is desirable.
It is an object of the present invention to provide a light guiding device that addresses one or more of the aforesaid issues.